While the following discussion and description will be provided in the context of traction kiting, this application is not intended to be limiting. The aerodynamic device of the invention may be used in other flying situations.
Unless the context requires otherwise, in the following description the terms “kite” and “wing” are interchangeable and refer to an aerodynamic device that is connected to its operator and/or their ski, skis, skates, board, buggy, or boat, by two or more lines, henceforth called flying lines. These flying lines restrain the kite, allow it to be manoeuvred in the air, and by their extension or shortening relative to each other can be used to effect control of the kite by the operator.
Traction kiting, where a kite is used to propel the kite's operator across the surface of the land, ice, snow, or water is becoming increasingly popular. The flier or operator of the kite dons a ski, or board, or pair of ski's, or skates, or stands on a board, or sits in a buggy or boat or kayak, whichever suits the type of surface and the style of traction kiting they wish to indulge, and uses the kite to harness energy from wind in order to propel themselves and their ski, skis, skates, board, buggy or boat across the surface of the land, ice, snow, or water.
Kite designs may be broadly characterised according to their construction. The range of constructions covers a broad spectrum from framed, semi-framed or unframed single skin kites to double skinned, or “ram air” type kites, to hybrid combinations of these various types. Regardless of their construction type, all kites must have some means of maintaining their spanwise shape while flying.
Four ways of forming or contributing to the form of the spanwise shape of a wing while it is flying are:                Firstly, the use of a rigid or semi-rigid frame.        Secondly, to contrive for air pressurised by being taken from, at or near a flow stagnation point to bleed into internal spaces within the wing, which pressurised air then functions as a structural element. This is referred to as the ram air system after Jalbert (see for example U.S. Pat. No. 3,285,546).        Thirdly, by arranging for some elements of the wing's aerodynamic surfaces to be otherwise than perpendicular to the general axis of the flying lines and set so as to generate aerodynamic forces that cause the wing tips to pull away from each other.        Fourthly, by the use of multiple bridle lines attached to the wing at intervals spanwise and chordwise and converging to the flying lines at a point or points between the operator and the wing. Such bridles, by distributing the tension in the flying lines more evenly over the surface of the wing, reduce the bending load on spanwise structural elements and therefore assisting in the retention of spanwise form.        
In the case of kites with rigid or semi-rigid frames, multiple bridles make it possible for these frame elements to be proportionally smaller and lighter, both of which are advantageous to the kite's performance by reducing respectively, drag and weight.
In the case of conventional Jalbert or Parapent style foil kites that use ram air inflation as their structural element, the pressure differential available is so small as to allow no possibility of sufficient spanwise beam strength without support from multiple bridles spaced at intervals spanwise. Because more bridles allow thinner and more aerodynamically efficient aerofoil sections to be used and also permit higher aspect ratio form thereby reducing induced drag there has been a tendency in recent years for parafoil style traction kites to have upwards of 60 bridles.
In the case where aerodynamic forces are used to retain or assist in the retention of spanwise form, multiple bridles reduce the proportion of the kite's aerodynamic surfaces that are required to be other than approximately perpendicular to the flying lines and hence increase the proportion of surface area that can be applied directly to supplying pull on the flying lines. An advantageous consequence of this can be a higher lift coefficient, which manifests as more pull in proportion to overall size.
Bridle lines do however, in themselves, add undesirable drag and can tangle during launching or flying in such a way as to prevent satisfactory operation of the kite.
Traction kites using various combinations of these four contributions to spanwise shape are known and used. Each have inherent advantages and disadvantages by cost, tangle resistance, luff resistance, power for size, upwind efficiency, packing ease, relaunch ease (especially from water), gust responsiveness and other values.
An example of a traction kite using mainly a combination of multiple bridle lines and aerodynamic forces to hold spanwise form is described in WO99/59866. This kite has a very flexible spar or bundle of spars comprising the leading edge of the kite and multiple panels, separated from each other by sets of bridles arranged in the flow wise direction and with these panels arranged and shaped so that the aerodynamic pressure distribution around them provides the major contribution to the spanwise form for the kite.
An example of a traction kite using a combination of only aerodynamic forces and multiple bridles to hold its spanwise form and with no rigid, semi-rigid or ram air structural elements is what has become known as the NASA wing. This kite has a single skin, shaped, and supported by bridles in such a way as to generate aerodynamic forces that are sufficient to form the spanwise and chordwise shape of the kite.
An example of a traction kite using ram air inflation as its spanwise structural element in combination with multiple bridle lines is described in Schimmelpfennig (U.S. Pat. No. 5,033,698). This kite is a double skin ram air inflated envelope with aerofoil profile ribs, without any frame but with multiple bridles distributed spanwise over the surface of the kite, and primarily near to the kite's leading edge, to assist retention of spanwise form. It does not use any significant contribution from aerodynamic forces to hold its spanwise shape beyond a small degree of “arch” form as is commonly used in foil type kites to offset the spanwise components of bridle tension that derives from their convergence to the flying lines.
An example of a traction kite using ram air inflation in combination with a spanwise semi-rigid structural element and some contribution to spanwise form from aerodynamic forces but without multiple bridling is described in U.S. Pat. No. 4,363,458. This kite is a double skin ram air inflated aerofoil with a semi-rigid spar spanning its leading edge. Two only flying lines are used, one attached to each wing tip at the leading edge. The trailing edge of the kite is unsupported by bridles or flying lines.
An example of a traction kite using a semi-rigid frame in combination with a single skin and spanwise aerodynamic forces but without multiple bridles or at least with very few bridles is described in U.S. Pat. No. 4,708,078. This kite has a structure of inflated tubes covered by a flexible skin. The tubes can be inflated through one or more sealable orifices using an inflatable boat pump for example to pressures significantly greater than would be available by ram air inflation. These tubes form a semi-rigid spar along the leading edge of the kite and usually also a series of flow wise orientated spines extending from the leading edge to the trailing edge at intervals across the span of the kite. The tubes must remain correctly inflated to maintain the shape required for the kite to fly satisfactorily.
To date there has been no disclosure of a kite that successfully uses the combination of only ram air inflation and aerodynamic forces to hold its spanwise form.
It is an object of the present invention to provide an improved aerodynamic device, which may be flown as a kite, including as a traction kite, which requires neither rigid nor semi-rigid structural elements spanwise and that, additionally, does not require the use of bridles distributed spanwise, or at least to provide the public with a useful choice.